During diagnostic testing of internal combustion engines, such as those commonly found in automobiles or boats, it is important to be able to detect the firing of the spark plugs and compile this information. Accurate compilation of spark plug firing gives an indication of engine RPM and is often essential to fast diagnosis of engine performance. Moreover, measurement of engine RPM coupled with other information such as vehicle velocity can provide performance data regarding the engine and vehicle.
The accurate monitoring of the firing spark plugs generally necessitates an electronic recording device. However, the extremely large electromagnetic and electrostatic fields produced by a firing spark plug can cause electronic recording devices containing microprocessors to malfunction. The present invention is directed to avoiding the foregoing and other problems by providing an apparatus that uses an optical fiber to isolate an electronic recording device from electromagnetic and electrostatic interferences.